Does anyone know why this is subjunctive? Could ‘que el viene’ still be correct?
It is practically a rule that the subjunctive is used in all subordinate clauses referring to a future event. The indicative could possibly work in a general statement: “suelo esperar aquí hasta que él viene”, but not in this context.
Ask yourself, is it possible that ‘he’ doesn’t turn up?
Remember, the indicative mood is used to express reality, the subjunctive dabbles in the ‘what might be’.